I heard the phrase on the radio "light is eternal" in the context of a subject that is not allowed under the TOS. It got me wondering if that is in some way true. It's certainly long lived, considering that astronomy deals with millions of light years, but eternal?

I will give an uneducated "Sort of".A photon is a packet of energy that would go on forever, if it never encountered anything else. I think photons can be absorbed and/or changed into something else though, they are not eternal in that sense. Photons don't experience any time though....since they move at the speed of light, even the oldest ones we see from billions of years ago, no time has passed from their perspective.

And so my image continues on even after I am gone. Such is immortality for we humans. OTOH, I can still remember an episode from a Superman comic of my past where Superman flew faster than the speed of light with a mirror and so was able to view events from the distant past directly by reflecting the light from his superluminal mirror. I remain very impressed by this fictional event. I believe if we could travel faster than light then, according to general relativity, we could actually travel back in time and change the past, which is why we can't travel faster than light.

I believe if we could travel faster than light then, according to general relativity, we could actually travel back in time and change the past, which is why we can't travel faster than light.

The barrier to such time travel isn't simply the difficulty of finding a loophole in the relativity principle permitting travel faster than c, but also entropy, which is an even more insurmountably forbidable barrier to time travel. While stargazing is backward "time travel" in the sense that you're seeing celestial objects as they were at the sometimes-formidably-ancient past time it left th eobject, that's nevertheless only a time-frozen artifact of that past time like a Matthew Brady photo of a civil war soldier. Having a copy of Brady's photo does *not* mean you thereby have any means to go back in time and speak or interact with the soldier in that photo, any more than you can have a conversation with a photo of yourself at age 8. Entropy is a more powerful force even than relativity in preventing you from doing so.

I believe if we could travel faster than light then, according to general relativity, we could actually travel back in time and change the past, which is why we can't travel faster than light.

But this argument turns on a conceptual puzzle, not a physical difficulty. Why not reason that we did change the past, and therefore we will come to be able to travel back so as to have done that? And why think we changed the past? Our existence depends on such remarkable circumstances that they must have come about by the design of someone with our interests in mind.

*ahem* Let me repeat - there is nothing weird about the statement that light is eternal. It's a BS comment because it's poetic and invokes timespans that do not exist. In fact light is, in reference to time, now and now alone - it goes from A to B in such a way that the proper time along this interval is not eternity, but zero. For all light, everywhere. Zero.

The only sense in which it is sensible is in reference to the non-interaction of photons with each other. Once emitted, light is independent of the source that did the emitting.

I believe if we could travel faster than light then, according to general relativity, we could actually travel back in time and change the past, which is why we can't travel faster than light.

But this argument turns on a conceptual puzzle, not a physical difficulty. Why not reason that we did change the past, and therefore we will come to be able to travel back so as to have done that? And why think we changed the past? Our existence depends on such remarkable circumstances that they must have come about by the design of someone with our interests in mind.

The last sentence of this post is not only nonsensical, but has virtually no logical relationship to the first three.

*ahem* Let me repeat - there is nothing weird about the statement that light is eternal. It's a BS comment because it's poetic and invokes timespans that do not exist. In fact light is, in reference to time, now and now alone - it goes from A to B in such a way that the proper time along this interval is not eternity, but zero. For all light, everywhere. Zero.

The only sense in which it is sensible is in reference to the non-interaction of photons with each other. Once emitted, light is independent of the source that did the emitting.

I believe if we could travel faster than light then, according to general relativity, we could actually travel back in time and change the past, which is why we can't travel faster than light.

But this argument turns on a conceptual puzzle, not a physical difficulty. Why not reason that we did change the past, and therefore we will come to be able to travel back so as to have done that? And why think we changed the past? Our existence depends on such remarkable circumstances that they must have come about by the design of someone with our interests in mind.

The last sentence of this post is not only nonsensical, but has virtually no logical relationship to the first three.

Well, I thought it was clear, but I'll be more explicit. I'm saying that the argument above is not correct. That argument I take to be:

Suppose we can (someday) travel faster than light.
Assume that if we can travel faster than light, then we can travel back in time and change the past. [I am not commenting about this assumption.]
So, it follows that we can travel back in time and change the past.
Assume that if we can change the past, we will change it.
So, it follows that we will change the past.
If we change the past, things are different than they are.
So, it follows that things are different than they are.
But it is impossible for things to be different than they are.
Since the supposition above leads to an absurdity, it must be false.
Thus, the conclusion follows that we can never travel faster than light.

Whether or not this argument is exactly what was intended, and whatever you may think of other parts of the argument, I am saying that the step "If we change the past, things are different than they are," is wrong. It's no more reasonable than its negation: "If we change the past, things are the same as they are." That's because our present results from our past, whatever it is, changed as a result of time travel or not changed.

To illustrate this, I adapted the classic argument from design. Suppose in a million years or so, we discover a way to travel back in time and change things, presumably the first thing that would occur to us is to ensure our own existence by going back to adjust natural constants, or whatever, as required. The clue that this has actually happened would be if we find that the early universe is suspiciously fine-tuned to make it possible for us to exist.

This reasoning is adapted from a Sci-Fi story I read several decades ago, whose author I forget, whose basic idea is that we exist in a time loop created by our descendents.

The reason you can't go back in time is that you can figure which stocks to buy, which horses to bet on etc. and get rich beyond belief, which would violate the capital asset pricing model and market efficiency.

I believe if we could travel faster than light then, according to general relativity, we could actually travel back in time and change the past, which is why we can't travel faster than light.

But this argument turns on a conceptual puzzle, not a physical difficulty. Why not reason that we did change the past, and therefore we will come to be able to travel back so as to have done that? And why think we changed the past? Our existence depends on such remarkable circumstances that they must have come about by the design of someone with our interests in mind.

The last sentence of this post is not only nonsensical, but has virtually no logical relationship to the first three.

Well, I thought it was clear, but I'll be more explicit. I'm saying that the argument above is not correct. That argument I take to be:

Suppose we can (someday) travel faster than light.
Assume that if we can travel faster than light, then we can travel back in time and change the past. [I am not commenting about this assumption.]
So, it follows that we can travel back in time and change the past.
Assume that if we can change the past, we will change it.
So, it follows that we will change the past.
If we change the past, things are different than they are.
So, it follows that things are different than they are.
But it is impossible for things to be different than they are.
Since the supposition above leads to an absurdity, it must be false.
Thus, the conclusion follows that we can never travel faster than light.

Whether or not this argument is exactly what was intended, and whatever you may think of other parts of the argument, I am saying that the step "If we change the past, things are different than they are," is wrong. It's no more reasonable than its negation: "If we change the past, things are the same as they are." That's because our present results from our past, whatever it is, changed as a result of time travel or not changed.

To illustrate this, I adapted the classic argument from design. Suppose in a million years or so, we discover a way to travel back in time and change things, presumably the first thing that would occur to us is to ensure our own existence by going back to adjust natural constants, or whatever, as required. The clue that this has actually happened would be if we find that the early universe is suspiciously fine-tuned to make it possible for us to exist.

This reasoning is adapted from a Sci-Fi story I read several decades ago, whose author I forget, whose basic idea is that we exist in a time loop created by our descendents.

OK, it appeared that you were defending the idea, not contradicting it. Maybe that's my poor reading ability.

" But it is impossible for things to be different than they are.
Since the supposition above leads to an absurdity, it must be false."

The problem with this statement is that it assumes knowledge not in evidence. We don't know everything about how things are now, so we can't say what is changeable and what isn't. It also denies both the dynamism and the stability of most natural systems: assuming you could go into the past and change something, that doesn't necessarily mean it would stay changed. The "butterfly effect" may influence the weather some weeks from now, but it isn't going to have any affect whatsoever on climate. That takes a much more drastic perturbation. Chaos theory is a theory of stability just as much as it is of changeability. In a system where you have an uncountably huge number of uncalculated permutations you can't say whether adding one more is going to make any long-term difference or not.

So prohibiting time travel on that basis seems flawed. (I'm not promoting time travel at all, I just think that we should stick to the arguments and points that are actually valid). If that is your thinking as well, then we agree after all.

As for the argument from design, the one in the above post seems more inspired by "Groundhog Day" than any particularly cogent hypothesis. I don't know who the author you're referring to is either, but I just don't see any concept there worthy of serious consideration. The entire "fine-tuning" argument is a farce from the get-go. It's just one more example of assuming your conclusion.

I think this might be my last time reading this specialty forum. It seems too often to be a place for certain types of people to push either their religious or their crackpot ideas.

popular mechanics and discovery channel trained "scientists"

So, since you folks raise the issue, what are your training and qualifications in science? And if they are eminent and distinguished, what are you doing spending your time arguing with non-scientists on this amateur site? Don't you have any bigger fish to fry?

I think this might be my last time reading this specialty forum. It seems too often to be a place for certain types of people to push either their religious or their crackpot ideas.

popular mechanics and discovery channel trained "scientists"

So, since you folks raise the issue, what are your training and qualifications in science? And if they are eminent and distinguished, what are you doing spending your time arguing with non-scientists on this amateur site? Don't you have any bigger fish to fry?

I think this might be my last time reading this specialty forum. It seems too often to be a place for certain types of people to push either their religious or their crackpot ideas.

popular mechanics and discovery channel trained "scientists"

So, since you folks raise the issue, what are your training and qualifications in science? And if they are eminent and distinguished, what are you doing spending your time arguing with non-scientists on this amateur site? Don't you have any bigger fish to fry?

This is spoken to ALL here in general, and NOT to any poster in this thread in particular:

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